Notes… • Handouts available for today and yesterday • Some bits might not come out too well, if you want to check against the originals I will put them all on my website: http://www.star.bris.ac.uk/katy • The versions on there currently are 2007 so please be careful!
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Notes… Handouts available for today and yesterday Some bits might not come out too well, if you want to check against the originals I will put them all.
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Notes…•Handouts available for today and yesterday
•Some bits might not come out too well, if you want to check against the originals I will put them all on my website:
http://www.star.bris.ac.uk/katy
•The versions on there currently are 2007 so please be careful!
Why are peculiar velocities useful? •Measure for a number of clusters in
a particular redshift ‘bin’ and minimise errors
•Repeat for a range of redshift bins•Can derive something about the formation of large scale structure - i.e. how quickly things are moving around at different redshifts
•Clusters move under gravity - learn about distribution of matter at different epochs
Surveys: New Science•SZ selected samples will allow us to
improve on ‘traditional’ SZ applications (Hubble const. etc)
•New frontier - cluster number density and its evolution with time
•The potential of this application will be realised with the release of cluster catalogues from SZ surveys
•One aim is simply to record how many clusters are found in e.g. different redshift bins
•Examine cluster evolution (e.g. mass functions) and the geometry of the Universe
Cluster Abundance
Distinguish between cosmological models
Carlstrom et al 2002
SZ-selected samples
•Previous SZ samples are often chosen somewhat arbitrarily - i.e. clusters picked because they are easy to observe
•Some attempts to select representative samples from X-ray catalogues (e.g. Jones et al 2005, Lancaster et al 2005)
•Still subject to ‘selection effects’ (i.e. X-rays point preferentially to dense clusters)
•SZ catalogues will be mass-limited only
SZ-selected samples•X-ray catalogues are limited in
numbers due to rapid fall off of detectable flux with distance
•SZ catalogues do not suffer from this limitation - will yield large numbers of new clusters, enabling studies of large scale structure via methods currently applied to galaxy catalogues e.g. 2DF
•Will also provide the first picture of the high-redshift Universe
SZ Science to date•Distance estimates to
reasonable precision
-Good agreement between different experiments
•ICM properties e.g. gas fractions
-Large errors but consistent between experiements
Future Science Prospects
•Detailed images - physics of clusters as individuals, and Universal population
•Large samples - more statistically robust estimates of cosmological parameters
•Blind surveys - direct view of the growth of large-scale structure over entire redshift range
SZ Practicalities•SZ is a tiny signal - requires
sophisticated observing techniques
•Various sources of contamination and confusion, which observing techniques deal with in different ways
-Radio sources (galaxies, planets)-Atmospheric emission, ground emission-Primordial CMB fluctuations•Today, a few details. We will discuss the various observing techniques and how they cope with these issues tomorrow
Radio Sources•If a radio source is present in the field of a galaxy cluster, it will ‘fill in’ the SZ decrement
•This could be true for sources in front of / behind the cluster, or indeed member galaxies
•Problem greater at low frequency: most sources are ‘steep spectrum’
•Can choose to observe clusters with no sources - introduce bias
•Better to ‘subtract’ effects•No high-freq. radio surveys - further complication
Atmosphere, Ground•Atmosphere is ‘warm’ - radiates.
•Time variable emission•Ground also a source of thermal emission
•Varies with pointing angle or telescope
•Can minimise this using a ‘ground shield’
Various ways exist of dealing with these contaminant signals
Primordial CMB•Primordial anisotropies look remarkably similar to the SZ effect on large angular scales (tens of arcminutes)
•Seem unsurprising that telescopes such as the VSA and CBI (built to observe the primordial CMB) suffer drastically from this type of contamination....
•.....We were still surprised!
Last Lecture.....
•More on the practicalities of observing the SZ effect - telescopes and observing techniques